A motor control device that drives and controls an alternating-current motor includes a converter that converts alternating-current power into direct-current power and an inverter that switches the direct-current power converted and generated by the converter to convert the direct-current power into alternating-current power for motor driving. In the inverter, however, noise is generated by switching operations of semiconductor switching elements. This noise is conductive noise that propagates to outside via a power line and causes influences on peripheral devices.
Therefore, conventionally, there has been a technique in which a noise filter is provided on a connection terminal of a power line of a motor control device to reduce leakage of generated conductive noise to the power line (for example, Patent Literature 1).
In the international EMI standards related to conductive noise generated in the motor control device, the leakage amount of conductive noise to the power line is regulated within a bandwidth of 150 kilohertz to 30 megahertz. The bandwidth of 150 kilohertz to 30 megahertz is normally a frequency range much higher than a bandwidth of a carrier frequency used for causing an inverter of a motor control device to perform switching operations with PWM signals.
Meanwhile, a noise filter has a configuration of a low-pass filter that passes a signal without attenuating it in a low-frequency area, and the cutoff frequency (frequency at which attenuation begins) of the noise filter is normally lower than a carrier frequency of a motor control device; however, manufacturers of such noise filters design noise filters while paying attention only to attenuation values around 150 kilohertz, but without paying much attention to the cutoff frequency thereof.
That is, the attenuation rate and the cutoff frequency of noise filters are different depending on the noise filter manufacturers, and thus, when such noise filters are combined with settings of different manufactures and users of motor control devices, the following problem occurs.
Specifically, when the amount of conductive noise generated by a motor control device is large, manufacturers and users of motor control devices select and use a noise filter having a large noise attenuation rate at a bandwidth of equal to or larger than 150 kilohertz. Meanwhile, as for noise filters designed by noise filter manufacturers having a large noise attenuation rate at a bandwidth of equal to or larger than 150 kilohertz, there is a case where the cutoff frequency thereof is made close to the carrier frequency of a motor control device.
When such a noise filter is incorporated in a motor control device that generates large conductive noise, in the motor control device, magnetic saturation is caused in a ferrite core of a common mode coil functioning as an inductor constituting the noise filter, and thus there is a case where a desired noise attenuation effect cannot be obtained.